1. Field of the Invention
The present invention relates to a Light-Emitting Diode (LED) driving circuit and LED driving control unit thereof; in particular, the present invention relates to an LED driving circuit and LED driving control unit for maintaining small amount of current and clamping voltage of the LED driving circuit.
2. Description of Related Art
Refer first to FIG. 1, wherein a conventional Light-Emitting Diode (LED) driving device is shown. The illustrated LED driving device comprises a dimming control unit 10, an error amplifier 15, an N-type Metal Oxide Semiconductor Field Effect Transistor (MOSFET) 20, a power supply 25, an LED module 30, and a current detection resistor R. The power supply 25 is coupled to one end of the LED module 30 so as to provide a driving voltage enabling the LED module 30 lighting. The N-type MOSFET 20 is coupled to the other end of the LED module 30, which controls the magnitude of current flowing through the LED module 30 based on a switch control signal. The current detection resistor R is coupled to the N-type MOSFET 20, which detects the magnitude of current flowing through the LED module 30 and generates a current detection signal. The error amplifier 15 receives the current detection signal and a control signal from the dimming control unit 10, thereby accordingly outputting the switch control signal to the N-type MOSFET 20 for controlling the magnitude of current passing through the N-type MOSFET 20. The dimming control unit 10 receives a dimming signal, and, when such a dimming signal represents an ON state, generates a reference level signal as a control signal such that the current detection signal and the reference level signal are equal in terms of electric potential, allowing the N-type MOSFET 20 to be in a conductive state; on the other hand, if such a dimming signal represents an OFF state, the dimming control unit 10 generates a signal of low level as the control signal, causing the N-type MOSFET 20 to enter into a cut-off state, thereby achieving the desired dimming effect.
Upon occurrence of the N-type MOSFET 20 being cut-off, current no long flows through the LED module 30, and at this moment, the electric potential at the drain of the N-type MOSFET 20 is pulled up and approximately equal to the driving voltage provided by the power supply 25. Taking 4 Volts of LED driving voltage for example, in case that the LED module 30 is composed of 20 LEDs connected in series, the driving voltage is 80 Volts and the N-type MOSFET 20 needs to be the high voltage MOSFET. The high voltage MOSFET can withstand high voltage, whereas occupies comparatively larger chip area, resulting in more production cost and significantly increases the gate-source capacitance (Cgs) as well. The increase in Cgs also leads to a fact that the error amplifier 15 is required to have higher driving capability in order to drive the N-type MOSFET 20. Furthermore, the power consumption of MOSFET is proportional to fCV2, where f indicates switching frequency, C indicates gate-source capacitance and V indicates electric potential change in switching operations. Consequently, the MOSFET having better high voltage withstanding capability consumes more power, and additional circuits are needed for heat sinking so as to prevent possible over-temperature or damage problems in the circuit and MOSFET.